As long as you always fill the tank exactly the same, and ride under the exact same conditions with the same fuel, and don't vary from those things, then the 1950's tripmeter method might be OK. Only thing is, back when that was the only method available we also had that handy dandy 1 gallon reserve on the petcock so if we didn't remember to reset the tripmeter, or didn't get the tank completely filled, or rode hard into a headwind so our tripmeter method was way off, we still didn't run out of gas because we had the reserve. OK, now lets move to the modern fuel injected era. There is no reserve fuel supply, so if your estimation using the tripmeter is off you could easily be either pushing your 700-900 pound bike down the side of the road, or sitting on the side of the road waiting for a road service truck with a can of gas. Not so romantic or fun these days, is it?
Get the gauge fixed. There is no excuse for an inaccurate gauge, and just because it's a motorcycle doesn't make an accurate gauge impossible to achieve. As I said before, as long as the majority of Harley riders let Harley screw them over on this issue, Harley will be more than happy to let things go along just as they are. If your dealer says it can't be fixed or adjusted, tell them they are FOS and then contact the MoCo. File a complaint on the NHTSA website, since it's a safety related defect. Raise a little hell. If enough of you do it, something will change.
Jerry